This invention relates to p-n junction structures, and more particularly, to a p-n junction structure whose electrical characteristics are affected by the application of pressure or other mechanical stresses.
The p-n junction, such as diodes and transistors are found in a widespread of applications, including structures which are affected by pressure or mechanical stresses. These devices have found a multitude of applications in the field of electronics ranging from the rectification of signals to the amplification of signals. The devices have also been used to convert one form of energy into another as, for example, the case of a solar cell where light is converted into electrical energy. New applications are continuously being found for p-n junction devices, which include the conversion of mechanical or acoustic signals into electrical signals. P-n junctions have particular applicability in the field of pressure and strain transducers because they have the advantage of static property measurement, their sensitivity is competitive with that of other signal devices, and they are inherently small in size and simple in construction.
The devices are well known. See for example the text entitled, xe2x80x9cPhysical Acousticsxe2x80x94Principles and Methodxe2x80x9d, Warren P. Mason, editor, published by Academic Press (1964), Chapter 12 entitled, xe2x80x9cUse of p-n Junction Semiconductor Transducers in Pressure and Strain Measurementsxe2x80x9d, pages 237 through 319 by M. E. Sikorski of the Bell Telephone Laboratories. Basically, this chapter discusses the use of semiconductor diodes in many different applications including strain gages, microphones and other devices as well. It is well known that in a multi-valley semiconductor such as silicon, the effective mass varies with direction being greater in some directions than others, the shape of the energy-momentum surface is an ellipsoid rather than a sphere. The effective mass m* is given by       m    *    =      1                            d          2                ⁢        E                    dp        2            
where E is the energy and p the momentum. For n type silicon, for instance, the mobility (xcexc), which is directly proportional to the effective mass, is much greater in the direction normal to the longitudinal direction of the ellipsoid and much smaller in the direction along the longitudinal axis. See FIG. 3, for example, showing a form of the energy surface which is plotted against the wave vector for silicon along the [100] axes. As one can see, the above-noted equation is also shown in FIG. 3.
In any event, for n-type silicon there are three sets of ellipsoids located on the three axes  less than 100 greater than  in energy momentum space. Reference is made to FIG. 4, where there is shown a multi-valley structure for n-type silicon in momentum space. As one can ascertain, there are three ellipsoids in energy momentum space and these are in the so-called  less than 100 greater than  axes. These ellipsoids are shown as 30, 31 and 32. The application of certain stresses will move some electrons from one ellipsoid to another, creating either more or fewer low effective mass electrons. If more low effective mass electrons result from the application of an external pressure or stress, the resistivity will decrease.
Similarly, for p-silicon, there are also three such ellipsoids that are situated along the various  less than 111 greater than  axes. In this case however, a tensile stress in the  less than 111 greater than  direction produces more heavy holes, thus increasing the resistivity. When one considers the p-n junction under mechanical stress, the effect of the change in the number of light and heavy carriers become very significant, particularly since current flow across the junction results from both holes and electrons.
It is also well known that the application of the stress can change the energy gap of a semiconductor, the resulting change depending upon the magnitude of the stress, as well as the direction with respect to the crystal orientation. This is true for both homogeneous, as well as porous structures, although in porous structure, this effect may be enhanced by Quantum Confinement. Moreover, since in a Zener diode or a tunnel diode, the junction current which also depends on effective mass will be changed. For the Zener diode, the reverse current will increase and the breakdown voltage will decrease, if the number of low effective mass carriers is increased. Similarly, for a tunnel diode, a greater number of low effective mass carriers will increase the current in the forward direction.
For a p-n Zener diode in the reverse direction, the current flow results mainly from the minority carriers in the less highly doped section of the structure. Thus, its crystallographic orientation with respect to any stress field is of paramount importance. Similarly, for a tunnel diode it is important to consider the crystallographic orientation of the structure causing majority carrier flow.
In any event, based on such considerations, it has been determined that by utilizing a p-structure and an n-structure, which are bonded together and which structures have a crystallographic orientation in different directions or in different orientations, one can achieve a diode structure which operates with improved efficiency over those of the prior art.
There is disclosed a p-n junction diode structure whose electrical characteristics can be affected by the application of pressure or other mechanical stresses that will control sensitivity. The p-n junction consists of two different semiconductor materials, one being of p-type and the other of n-type, both having predetermined crystallographic axes which are fusion bonded together to form a p-n junction. Because of the ability to control the position of the crystallographic axes with respect to one another, one can affect the electrical characteristics of the p-n junction and thereby produce devices with improved operating capabilities such as Zener diodes, tunnel diodes as well as other diodes.